Extend behavior for g:bufferline_show_bufnr

Changed meaning of g:bufferline_show_bufnr to the minimal number of buffers required to show buffer numbers. 0 still turns off buffer numbers.
This is useful when user doesn't want buffer number to show up when there is only one buffer: Achieved by setting g:bufferline_show_bufnr to 2.
I don't think that anyone would use greater values, but anyway it's nice to have feature.